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CHROMOSOMAL ARRANGEMENT
Independent Assortment
Definition
Random orientation of homologous pairs
within a tetrad toward either pole during
metaphase I.
Purpose
Diversity of genetic combinations
CHROMOSOMAL ARRANGEMENT
Crossing Over
Definition
Physically attached homologous
chromosomes in a tetrad exchange DNA
segments at the chiasma during Prophase I
Process
Synapsis
Purpose
Increased genetic combinations
Chiasma
Fig. 23.14b
CHROMOSOMAL ARRANGEMENT
Chromosome Nondisjunction
Definition
1) Failure of homologous pairs to separate
during Anaphase I
2) Failure of sister chromatids to separate
during Anaphase II
Fig. 26.2b
Fig. 26.2a
NONDISJUNCTION
Example: Down’s Syndrome
Nondisjunction
Homologous pair of chromosome 21
fails to separate during Anaphase I in
Primary Oocyte.
Results
Fertilization yields three number 21
chromosomes.
DOWNS SYNDROME
Trisomy 21, XX, +21
Jacob’s Syndrome, 47, XYY
Genetic Terminology
Gene Locus
Definition
A gene’s particular location on a chromosome
Nomenclature
7q31.2 or 7p31.2
7 = Chromosome number
p = short arm
q = long arm
31.2 = G band address of loci
GENE LOCUS
GENE LOCUS
Genetic Terminology
Allele
Definition
One of two or more slightly different molecular forms of
a gene that code for different versions of the same trait.
Types
Dominant (B)
Allele expressed
Recessive (b)
Allele expressed only in the homozygote
Homozygous (BB or bb)
Two identical alleles
Heterozygous (Bb)
Two nonidentical alleles
Fig. 23.2a
ALLELES
Genetic Terminology
Genotype
Genetic makeup (alleles)
BB, bb, Bb
Phenotype
Physical appearance, behavior or physiological
activity controlled by genes
BB, Bb = BROWN
bb
= blue